1. Field of the Invention
In at least one embodiment, the present invention relates to a platinum-group-metal (“PGM”) free catalytic compositions and the use of such compositions as a catalytically active washcoat for regenerable catalyzed diesel particulate filter applications and for automotive after-treatment systems.
2. Background Art
Environmental regulations in the United States and Europe have necessitated improvements in the removal of particles from diesel engine emissions. Typically, such particles are carbonaceous particulates in the form of soot. Presently, the most promising method for removing soot from engine exhaust is by collecting the soot using a diesel particulate filter (“DPF”) followed by oxidation of the accumulated particulates at elevated temperatures.
Diesel particulate filters have been installed in urban buses and passenger cars as early as 1986. More recently, PSA launched a first European diesel passenger car having a particulate filter system. However, the regeneration by soot oxidation of diesel particulate filter requires elaborate solutions because the usually low temperatures of diesel exhaust gases are not favorable for soot oxidation. One solution is provided by supported or fuel-borne catalytic assistance in the regeneration of the DPF. A filter system which uses such a fuel-borne catalyst approach is complex and requires additional components such as a tank for fuel additives, an additive dosing system, infrastructure to refill the additive fuel tank, and the like. Moreover, fuel-borne catalysts lead to the formation of ash accumulated on the filter with gradual loss of filter soot capacity and a decrease in time between regeneration events. This phenomenon makes it necessary to change the filter after about 80K kilometers.
A soot filter which includes one or more catalysts is theoretically more attractive because it is potentially a less complex way to solve the problem of soot oxidation. However, presently available catalyst-containing filters still use PGM compositions, particularly platinum based formulations on different oxides (Oi-Uchisava J., Obuchi A., Enomoto R. Liu s., Nanba T. and Kushiyama S, Appl. Catal., B 2000, 26(1), 17–24; Oi-Uchisava J., Obuchi A., Ogata A., Enomoto R. and Kushiyama S., Appl. Catal. B 1999, 21(1), 9–17; JP 11253757).
Considerable effort has been devoted to the development of optimized PGM-containing diesel particulate filters, and in particular to Pt-containing diesel particulate filters. Engelhard has several patents on Pt-containing catalysed soot filters (WO 00/29726 A, EP 0 160 482 B1, U.S. Pat. No. 100,632, EP 0 164 881 A1, U.S. Pat. No. 5,100,632). Johnson Matthey (“JM”) is producing a commercially Pt-containing catalysts for a DPF-system called “Continuously Regenerating Trap” (“CRT™”). The Continuously Regenerating Trap comprises a diesel particulate filter for soot oxidation and a platinum-based diesel oxidation catalyst (“DOC”) positioned upstream of the diesel particulate filter because of NO2 generated on the Pt-containing DOC (Platinum Metals Review, 45(1), 2001, 30). Although particulate filters no containing catalysts have been used since 1996, JM later included Pt- or Pd-containing oxidation catalysts within diesel particulate filters to improve soot oxidation on the filter (WO 01/12320 A). PGM containing catalyzed DPFs have as well been described by Degussa A G (U.S. Pat. No. 4,900,0517; EP 1 055 805 A1). A PGM-containing diesel particulate-NOx reduction System (DPNR) designed for simultaneous removal of soot and NOx from diesel exhaust that will be launched in vehicles in 2003 has also been described. (Automotive Engineering International/October 2000, p.119; U.S. Pat. No. 5,746,989; EP 0 758 713 B1). In addition, several other patents deal with certain improvements concerning PGM-containing particulate filters/traps (EP 0 658 369 B1; U.S. Pat. No. 5,330,945; U.S. Pat. No. 4,759,918; U.S. Pat. No. 5,610,117; U.S. Pat. No. 5,911,961; U.S. Pat. No. 6,143,691)
The PGM contained in these DPFs, usually in the form of a catalytic coating are very expensive. The world demand on PGM use in automotive exhaust after-treatment is high, while PGM supply is limited. The high cost associated with PGM loadings in these compositions and an expectedly drastically increasing world demand on PGM supplies illustrates the necessity to search for economical solutions replacing the PGM in diesel particulate filters.
Furthermore, PGM coatings are highly active in undesirable reactions such as oxidation of SO2 to SO3, with the following formation of sulfated ash and sulfated particulate. To minimize this side effect, catalyst suppliers try to decrease the PGM concentration in the coatings. This, however, leads to significantly lower activities in soot oxidation and thus efficiency. In addition, PGM-containing washcoats/coatings are vulnerable to poisoning by sulfur compounds, particularly in the case of low Pt- or Pd- loading, respectively. Therefore the aforementioned CRT™ and DPNR filters are economically and in terms of efficiency restricted to the use together with fuel of very low sulfur level.
Advantages of PGM-free catalysed DPFs, therefore, would be generally lower costs, as well as better specificity, avoiding undesired side reactions. Also, the application of higher concentrations of the catalytically active components may become possible, thereby increasing the sulfur resistance of the catalytic coating/washcoat. The basic problem is to find an active catalyst, which would be able to replace PGM metals.
At present, there is no commercially applicable solution available to the problem of PGM-free catalysed DPFs, though attempts employing vanadium-containing catalysed DPFs have well been described. Degussa A G mentioned the possible application of vanadium-based washcoats (U.S. Pat. No. 4,900,0517; EP 1 055 805 A1). Also, Redem Corporation disclosed DPFs comprising coatings based on vanadium compounds (U.S. Pat. No. 6,013,599). Vanadium-containing catalysts on a porous ceramic carrier have been reported by Bridgestone Corp. (U.S. Pat. No. 4,711,870).
A copper-vanadium composition was proposed for application on filters (U.S. Pat. No. 5,340,548). Similarly, a Cu/V/K/Cl— based catalytic filter was developed, which was said to be especially active in the presence of NO in the exhaust gas (P. Ciambelli, V. Palma, P. Russo and S. Vaccaro, Stud. Surf. Sci. Catal. 1998, 116, 635–645; P. Ciambelli, V. Palma, P. Russo and S. Vaccaro EUROPACAT— IV, Rimini, Italy, 1999, Book of abstracts, P/I/337; P. Ciambelli, V. Palma, P. Russo and S. Vaccaro, Appl. Catal. B 1999, 22(1), L5–L10). These catalysts, however, exhibited no noticeable activity below 400° C., furthermore, they were not stable. A catalyst of the composition Cs4V2O7—V—AgCl—CsCl was reported to be active at about 370° C. (G. Saraggo, N. Russo. M. Ambrogio, C. Badini, V. Specchia. Catal. Today, 2000, 60, 33–41). Other Authors tested vanadium-based catalysts for soot oxidation (Carabineiro S. A., Bras Fernandes F., Ramos A. M, Vital J., Silva I. F. Catal. Today 2000, 57 (3–4), 305–312).
These vanadium-containing formulations though not being expensive, are highly toxic. Furthermore, their relatively low melting point leads to inevitable noxious vanadium emissions from the filter during running due to the high temperatures developed in the course of soot combustion. In addition, these formulations are not active at low temperatures below 350° C. Optional addition of PGM was mentioned for patents with vanadium catalysts. Among other catalysts, a very promising low temperature activity near 300° C. was reported for Cu—Nb(Ta)—K—La2O3 (TiO2), but Nb and Ta also are very expensive, annihilating any economical benefit. In addition, the stability of the catalyst is low due to the presence of chlorides (A. Bellaloui, J. Varloud etc. Catal. Today, 1996, 421–425).
The combination Co and K/MgO has been found to be active near 400° C. while the combination K and Co/La2O3 has been found active near 350° C. for soot combustion in the presence of NO(C. A. Querini, L. M. Cornaglia, M. A. Ulla, E. E. Miro, Appl. Catalysis B: 1999, 20, 165–177; E. E. Miro, F. Ravelli, M. A. Ulla, L. M. Cornagli, C. A. Querini, Studies in Surface Science and Cartalysis, 130, 2000 Elsevier, 731–736). FeCrAl foam, coated with Ce—Mn/SiO2—Al2O3, is reported to be active at 350° C. (EUROPACAT— IV, Rimini, Italy, Book of abstracts, W. Tylus. Metallic foamed filter-catalysts for oxidation of diesel soot, p/II/218, p.756). Molten salts catalysts have also been developed for soot combustion. However, these catalysts are not attractive due to their low stability and high corrosive activity (S. J. Jelles, B. A. A. L. van Setten, M. Makkee, J. AS. Moulijn. Appl. Catalysis B: 1999, 21, 35–49; Cat. Today 1999, 53, 613–621; Barry van Setten, Ph.D. Thesis, Delft University, 2001).
Accordingly, there exist a need in the prior art for non-PGM catalysts that oxide soot and for diesel particular filters which are active at low temperatures. In particular, there is a need for such catalysts and filters that are economical and efficient with improved ecological characteristics.